Recycling of scrap aluminum has grown substantially in recent years due to legislation and to efforts by the aluminum industry to reduce energy consumption and capital investment. About one half of scrap aluminum comes from mill wastes. The amount of scrap from used beverage cans, however, has grown rapidly causing a demand for new melting and refining capacity.
In order to improve the economics of recycling, substantial improvements have been made in the design and operation of aluminum melting furnaces. Better control of the temperature set point and combustion stoichiometry have improved fuel efficiency. Dross formation, i.e. formation of oxide on the surface of the aluminum during heating, has been reduced somewhat by improved operating practices. In direct-fired furnaces, however, further dross reduction has been limited by the presence of oxidative gases in the furnace atmosphere, particularly oxygen and combustion products that are emitted from the direct-fired burner. Specifically, the atmosphere of a direct-fired furnace contains CO.sub.2, H.sub.2 O and O.sub.2, to which the aluminum charge is constantly exposed. The combined concentration of CO.sub.2, H.sub.2 O and O.sub.2 is typically about 30% when air is used as the oxidant. Most dross formed during aluminum melting is believed to result from contact with these oxidizing gases. Although the effects of melt temperature, melt composition, and furnace atmosphere on the rate of oxidation are reasonably well understood, improvements in the amount of dross formed have been limited.
Use of a controlled atmosphere in an indirectly-fired furnace which heats the aluminum charge from radiant tubes can reduce oxidation loss substantially. But the reduced heat transfer rates, corrosion of the radiant tubes and high capital and maintenance costs of such furnaces make them uneconomical.
There is therefore a need for a direct-fired aluminum melting practice that results in substantially less oxidation and dross formation without substantially increasing capital or production costs.